1. Technical Field
This invention relates to automotive interior trim structures incorporating a cover for an air bag deployment opening and, in particular, relates to a method and apparatus for casting a tear seam into the decorative outer skin of an automobile interior trim structure by controlling the relative temperature of the portion of the mold used to form the tear seam in the outer skin. Further, the invention relates to a resultant cover and tear seam construction.
2. Description of the Related Art
Various types of supplemental involuntary restraint systems (SIRs) are currently being provided by automobile manufacturers to help reduce the extent of personal injuries incurred in automobile accidents. Air bags are increasingly becoming one of the most common and popular SIRs utilized. Air bags are designed to inflate during a collision to restrain forward movement the driver and/or other occupants to help avoid injurious contact with interior portions of the automobile. Driver's side air bags currently come as standard equipment on many models and optional passenger side air bags are becoming more common.
Air bags are typically stowed behind one or more interior trim structures, such as the steering wheel cover, door panel, or passenger side portion of the instrument panel. These interior trim structures must therefore be specially manufactured to permit deployment of the air bag upon a collision being detected. Such manufacturing entails consideration of a multitude of design requirements, such as the functional requirements of the air bag deployment system and the aesthetic requirements of the interior trim structures. Moreover, many of these requirements involve countervailing considerations. For example, the air bag should be able to be deployed virtually instantaneously upon a collision being detected. However, it should be stowed in such a manner as to 1) inhibit accidental or intentional tampering that would interfere with its performance and 2) contribute to an overall pleasing interior appearance. In regard to the latter consideration, it is as a matter of human psychology, undesirable to remind the automobile occupants of the dangers of driving and the existence of the air bag is therefore preferably made entirely invisible.
Interior trim structures used in automotive applications typically comprise a composite article having a foam layer formed between a rigid insert and a decorative outer skin. The insert provides structural reinforcement to the interior trim structure and is used to secure the trim structure within the automobile interior. Because the reinforcing insert is rigid, it either has an opening through which the air bag is deployed or is manufactured with one or more hinges which define one or more doors that open upon the air bag being inflated. The air bag is stowed behind this composite article until such time as it must be deployed.
In the typical air bag deployment scheme, the air bag forces its way out of its stowed position upon expansion. Various arrangements of the interior trim structures have been suggested to accommodate deployment of the air bag in this manner. For example, U.S. Pat. No. 3,640,546, issued Feb. 8, 1972 to D. S. Brawn, discloses various air bag enclosures, each of which essentially comprise a pair of doors secured together by a seal which is ruptured upon expansion of the air bag. The inflating air bag forces the doors to swing outwardly, thereby allowing the air bag to expand into the automobile compartment. In one of the disclosed embodiments, the doors are covered by an outer layer which is preweakened along a joint line. Although this patent states that the preweakening can be accomplished by reducing the thickness of the outer layer along the joint line, the only means disclosed for accomplishing that result is high frequency heating while loading.
U.S. Pat. No. 4,246,213, issued Jan. 20, 1981 to Y. Takamatsu et al., discloses a method for casting in V-shaped cut portions that extend through the foam layer and partially through the outer layer to define a tear seam along which the trim structure ruptures upon inflation of the air bag. The method utilizes two male mold members, each having projections which forms a part of the cut portion. The first mold member is used to form the outer layer and has one or more projections which form a groove in the back side of the outer layer. The second mold member is used to form the foam layer and has one or more projections arranged to extend at their tips into the groove previously formed in the outer layer. These projections form slits in the foam layer which are aligned with the grooves to thereby form the V-shaped cut portions.
Arrangements utilizing a continuous foam layer and outer skin are known. For example, U.S. Pat. No. 5,082,310, issued Jan. 21, 1992 to D. J. Bauer, discloses a closure for an air bag deployment opening which includes a pair of rigid, hinged doors covered by a foam layer and an outer skin. The doors are connected by a bridging portion which breaks upon inflation of the air bag. The outer skin is cut (i.e., grooved) along its inside surface proximate the bridging portion of the doors to define a tear seam along which it ruptures upon deployment of the air bag.
One problem that results from cutting or scoring a tear seam into the skin is that, at colder temperatures, the plastics used to form the outer skin become relatively brittle and the skin is therefore more susceptible to cracking along the tear seam. At the opposite temperature extremes, these plastics become relatively soft and deformable, thereby becoming more difficult to rupture. These offsetting considerations make it more difficult for manufacturers to produce an outer skin having a tear seam that remains unbroken when subjected to expected environmental conditions yet will rupture when needed.
A second disadvantage of cut tear seams is that they require a separate manufacturing step which must be completed after the outer skin is formed, but prior to co-molding the skin to the foam layer. Moreover, as shown in U.S. Pat. No. 5,082,310, discussed above, cutting the skin typically requires that it first be removed from the mold.
Another method for forming grooves in a thermoplastic material is disclosed in U.S. Pat. No. 4,886,630, issued Dec. 12, 1989 to T. Sugiura et al. This method, however, not only requires that the outer skin member be preformed in a separate operation, but also requires, in addition to the female mold member, a separate male mold member and a heated press member which extends through an aperture in the male mold member to form the grooves.
The aforementioned shortcomings of cutting and press-forming an outer skin to form a groove therein can be avoided by casting the tear seam into the skin during formation of the skin. Although U.S. Pat. No. 4,246,213, discussed above, shows one method of casting a groove into the tear seam, it requires a separate male mold member which must be inserted and removed from the female mold member during each manufacturing cycle.
Other, more suitable means of forming an outer skin are disclosed in U.S. Pat. No. 4,623,503, issued Nov. 18, 1986 to E. Anestis et al., and U.S. Pat. No. 5,032,076, issued Jul. 16, 1991 to K. L. Jackson, Jr., each of which is hereby incorporated by reference. U.S. Pat. No. 4,623,503 discloses a method and apparatus for casting an article by impinging heated air onto the backside of a mold to fuse together material disposed on the inner surface of the mold. U.S. Pat. No. 5,032,076 discloses a mold assembly having heat transfer pins located on the outer surface of a mold shell to facilitate the transfer of heat from air flowing about the outer surface to the inner surface of the mold.